Science

http://volcano.oregonstate.edu/

http://www.gp.com/EducationalinNature/index.html

Chem4Kids

Good basic to advanced information for all levels of students.

Biology4Kids

Good basic to advanced information for all levels of students.

Geography4Kids

Good basic to advanced information for all levels of students.

Physics4Kids

Good basic to advanced information for all levels of students.

NASA Education

Education site from NASA.  The Kids’ Club has a wide range of activities addressing all kinds of skills, including completing the pattern, etc. on the lowest level.

StarChild

This is a another great NASA resource for astronomy information and can be chosen by level. It is still pretty high but the plethora of pictures may be helpful.
Use Universal Reader to have it read independently to your kids with high receptive language.

Water Cycle Animation

Great talking website demonstrating the water cycle.

ARKive Education Resources

Awesome British site of all kinds of pictures and videos of our earth and its inhabitants

Miss Maggie

Great free resource of teaching ideas and packets (worksheets) that have different teaching materials for different levels of learners: emergent, primary, and intermediate.

BiteSize Science: Living Things

Awesome page of links to many different kinds of science activities for elementary level learners.